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The FBI today forcefully defended its decision not to criminally charge Hillary Clinton in connection with her use of a private email server as US Secretary of State in a letter to legislators that laid out its rationale for refusing to do so.

The letter was sent to House Oversight Committee members the same day that the bureau released to the GOP-controlled Congress a variety of materials from its investigation.

It marked yet another occasion in which FBI leadership responded to - and in some cases, rebutted - GOP claims about why the Democratic presidential nominee should have been charged.

But the materials turned over - which included an investigative summary; reports known as "302s" containing interviews with Clinton and others; and classified emails found on her private server - may give Republicans fodder to criticise Clinton in the months before the election.

That is, if that material - some of which is classified and marked "secret" - is actually released or leaked by Republicans, which could put them in the awkward spot of defending the release of sensitive information, which they have criticized Clinton for mishandling by using a private server as Secretary of State.

FBI Director James Comey announced in early July that he was recommending Clinton not be charged, and the letter, which was released today by Democratic Congressman Elijah Cummings, largely repeated statements he had made previously in public.

But it also notably seemed to take aim at some ongoing conservative criticisms of Clinton - particularly that she was negligent in her handling of classified information and thus deserving of criminal charges.

"As the Director stated, the FBI did find evidence that Secretary Clinton and her colleagues were extremely careless in their handling of certain, very sensitive, highly classified information," FBI Acting Assistant Director Jason Herring wrote.

"The Director did not equate 'extreme carelessness' with the legal standard of 'gross negligence' that is required by the statute. In this case, the FBI assessed that the facts did not support a recommendation to prosecute her or others within the scope of the investigation for gross negligence."

The documents released were turned over to several House committees, including Oversight and Government Reform, the Permanent Select Committee on Intelligence and the Judiciary panels. The bureau said the materials were provided to "assist [legislators] in their oversight responsibilities in this matter," and they were turned over "with the expectation it will not be disseminated or disclosed" unless the agency agrees.

An Oversight spokesperson said, "Committee staff is currently reviewing the information that is classified SECRET. There are no further details at this time."

Even with the letter, the delivery of the material to the Republican Congress in the midst of a heated campaign season is not likely to help the campaign of Clinton, who is leading in the polls against GOP presidential nominee Donald Trump. Polls show that 60 per cent of voters don't believe the Democratic nominee and former first lady is "honest".

Herring, who directs the FBI's Office of Congressional Affairs, wrote that someone else who engaged in the type of conduct of which Clinton was accused might face "severe administrative consequences," and the FBI was in the process of providing relevant information to other government agencies. The State Department, for instance, is conducting its own internal review of whether classified information was mishandled.

Their conclusions could affect future security clearances for possible members of a new Clinton administration who may have violated rules when they previously worked at Clinton's State Department.

But Herring also asserted that investigators found no evidence Clinton intended to mishandle classified information, noting that even three emails marked as classified did not originate with her and that State had determined two of them did not contain classified information.

Senate Intelligence Committee ranking member Dianne Feinstein, a Democrat of California, said: "The FBI's letter to the Judiciary Committee makes clear this is much ado about nothing".

Clinton campaign spokesman Brian Fallon called the document transfer "an extraordinarily rare step that was sought solely by Republicans for the purposes of further second-guessing the career professionals at the FBI".

"We believe that if these materials are going to be shared outside the Justice Department, they should be released widely so that the public can see them for themselves, rather than allow Republicans to mischaracterise them through selective, partisan leaks," Fallon argued.

Clinton has sought to defray controversy over the email server she used while serving as Obama's chief diplomat, saying using it was a "mistake" for which she is "sorry". But the issue continued to swirl as the FBI investigated whether there was criminal behaviour in the use of the server by Clinton and her then-aides at State.